Non-fluting printing substrate and method for producing the same

ABSTRACT

The present invention relates to a printed substrate comprising a substrate and a printing ink and is characterized in that at least one side of the substrate has a surface with such porosity that the Gurley-Hill air permeability value of the substrate is above 7000 s/100 ml, and it comprises an printing ink formulated as heat-set offset printing ink with reduced tack comprising at least one pigment and a mixture of at least two solvents boiling in the range of from 200° C. to 270° C.

The present invention relates to a printed substrate, and in particularto a printed paper. In terms of this invention substrate means to alsoinclude, but being not limited to, a paper. The printed substratecomprises at least one coating and a printing ink, formulated asheat-set printing ink. The printing ink has a reduced tack and comprisesat least one pigment and a mixture of at least two solvents boiling inthe range of from 200° C. to 270° C.

The term coating includes any surface treatment applied to at least oneside of the substrate resulting in reduced porosity of the respectivesurface of the side of the substrate.

In terms of this invention, the expressions “heat-set offset printinginks, including heat-set web offset (HSWO) printing inks” are always tobe understood such that the respective printing inks are formulated forheat-set offset printing, and hence do fulfil the properties beingnecessary as matter of principle for such printing inks.

In other words, the particular surface of the unprinted substrate can beachieved, for example but not limited to, by coating the substrate witha coating colour.

The porosity of the surface of the unprinted substrate is such that theGurley-Hill air permeability value of the substrate is above 7000 s/100ml, preferably above 10000 s/100 ml and most preferably above 12000s/100 ml.

The printed substrate also comprises a non-fluting printing ink beingformulated as heat-set printing ink. The printing ink has a reduced tackand comprises at least one pigment and a mixture of at least twosolvents boiling in the range of from 200° C. to 270° C. and furthermoreis designed to be printed onto the substrate during a heat-set offsetprinting process, in including, but not limited to, a heat-set weboffset (HSWO) printing process. The printing process is for achievinginter alia the desired and advantageous effect of considerably lessfluting in terms of AFT values being below 0.05%. According to preferredembodiments of the process described according to the present inventiongloss and smoothness of the printed substrate are also improved.

The present invention also relates to a method of manufacturing theprinted substrate by a heat-set offset, including a HSWO printingprocess. The term heat-set offset printing in the context of the presentinvention refers to heat-set offset printing making us of fountsolutions that are water-based. In terms of this invention heat setoffset printing means to include, however is not limited to, HSWOprinting.

The present invention still further refers to the use of a substrate anda printing ink being formulated as heat-set offset printing ink. Theprinting ink has a reduced tack and comprises at least one pigment and amixture of at least two solvents boiling in the range of from 200° C. to270° C. It furthermore may comprise also a low boiling solvent. Theprinting ink is designed for producing a printed substrate showingconsiderably less fluting in terms of AFT values.

Not intending to be bound by this theory: during a heat-set printingprocess, the printing ink is usually dried in high-temperature ovens. Inan early stage of the drying process of the printing process, moisturecontained in the coated paper evaporates rapidly from non-imaged areas,resulting in considerable shrinkage in the cross direction of the coatedpaper within this non-imaged area. On the contrary, moisture of thecoated paper located underneath an imaged area evaporates rather slowly,since the printing ink layer on the coated paper acts as a barrier toheat transfer and moisture evaporation, resulting in little shrinkage inthe cross direction of the coated paper within this imaged area. As aresult, the non-imaged area of the coated paper compresses the adjoiningimaged area in the cross direction during the drying process, leading tobuckling of the imaged area.

This phenomenon is known as “fluting” and it appears during heat-setprinting, in particular during heat-set web offset printing, and causescustomer complaints and various defects, such as for example waviness.

The problem is inter alia paper related and occurs due to the structureof the wood fibres forming the backbone of the paper web. Wood is aninhomogeneous compound of different fibres. These structures arestabilized by chemical bonds. Inside those fibres water molecules act asspacers, increasing fibre dimensions and affecting dimensions of thewhole paper web. During the printing process, the fibres pick up morewater and swell. In the hot air floatation dryer besides the printingink, also the paper is dried. The water molecules between/inside thefibres evaporate, allowing the fibres to shrink and to come into closecontact with each other. Thereby the paper web shrinks. New chemicalbonds are formed between these fibres. Thus the dimensions of the paperare altered. Since after the printing process, there are areas ofdifferent water content in the paper web (i.e. water content in theimaged and non-imaged areas) this alteration of the dimensions of theweb is more severe in some areas than in other adjacent areas. Due tothe different swelling and shrinking extent of the fibres in these areaswith different water content, waves form on the paper. The newly createdchemical bonds are so strong that they will not be broken downcompletely after re-moistening.

In other words, within this description, the term “fluting” is definedas undulating creases, waves or bands that form in the printed paperafter having passed a heat-set dryer. The corrugations appear in thedirection of web travel and are, as mentioned before, more or lesspermanent, i.e. they do not relax until months after product delivery.

Many efforts have been made in order to overcome the problem of flutingduring heat-set printing.

According to the prior art, new paper products are suggested, as forexample described in Jon Tappi J., (2003) vol. 57, no. 1, January 2003,pp 92-97. There are papers described (launched by Oji Paper) which donot show fluting in web offset printing. It is stated that contractileforces, resulting from drying after web offset printing are minimized.However, the use of such paper causes higher costs and in someapplications the publishers as well as the printer are limited inprinting ink choice.

Also, coatings capable of avoiding fluting are suggested in the art. InTappi J., (2000) vol. 83, no. 4, April 2000. It is described that thetendency to flute is to a great extent determined by the coatingformulation, wherein less absorbent coatings are supposed to decreasefluting. No concise suggestions are made regarding particular coatingsand printing inks being suited to be used thereon.

One should note that according to the prior art various concepts basedon different theories have been suggested to avoid the fluting problem.

During the 2002 (69th) Pulp and paper research conference, Tokyo, Japan,17-18 Jun. 2002 and in the respective report (P-06, pp 166-171 [Tokyo,Japan: Japan TAPPI, 2002, 186 pp]) it is described that dryingtemperatures in the range of 135° C. prevent fluting during offsetprinting processes. The drying conditions in an early stage of theprinting process are described to be important in the prevention offluting. The concept does not lead to reasonable results.

From the art the skilled person is confused about concepts that existfor solving the fluting problem, particularly occurring in heat-setprinting processes.

There are remoisteners on the market for all alleviation of fluting, butthey do not give reproducible results.

According to J. Pulp Pap. Sci., (September 1993) Vol. 19, no. 5,J214-219, the results of theoretical and experimental analyses arepresented, directed to suggestions for the alleviation of fluting bychanging in dryer design and operation.

U.S. Pat. No. 6,058,844 refers to a method of and an apparatusdiminution of fluting or corrugation occurring in printed webs of lightweight coated paper printed on both sides with thermosetting printingink on HMO printing presses. The problem is solved by spreading the webin its width-wise direction as the printed web exits the printing inkdrying and heat setting oven of the press and passes over the webcooling chill rolls. Thereby the printed web is held in a flat andsmooth condition until it is cooled down and the printing ink takespermanent set. Spreading the web prior to and during cooling allows theprinting inks to thermoset in a flat state, because the web is kept flatand free of flutes during thermosetting. The method and apparatus isdescribed to facilitate operation of the press at higher speeds and withlighter grades of paper. However, experience with such devices does notshow the indicated advantages.

Additionally, WO 2004/003293 describes a paper having some specificfeatures, with an oleophilic surface of the coating being the mostrelevant of these features and the gist of that invention. In order toachieve a suitable surface characteristic, which is mentioned in thisart, it is obligatory to use oleophilic substances on the surface of thepaper to be printed, like SMA based additives (Raisaprint D100 orRaisaprint D200, see page 5 of WO 2004/003293). They are used to controlsurface chemistry and to achieve the oleophilic character of thesurface. Coating paper with these substances is expensive and thereforenot favorable.

U.S. Pat. No. 5,713,990 and U.S. Pat. No. 5,875,720 describe printingink compositions comprising high boiling oils as solvents in theprinting ink vehicles. Also, the bodied tung oil described as solvent inU.S. Pat. No. 6,206,960, which is present in the printing inkcomposition, decomposes only at temperatures greater than 350° C. Thedrying temperatures suggested during the printing process described inU.S. Pat. No. 6,206,960 are as high as 149° C. U.S. Pat. No. 6,709,503discloses modified linseed oil as solvent in printing ink compositions,which solvent decomposes only at temperatures greater than 350° C. Thedrying temperatures during the printing process are described to behigh. U.S. Pat. No. 5,427,615 introduces fatty acid ester solvents withhigh flash points, decomposing only at temperatures above 350° C.

U.S. Pat. No. 4,357,164 disclose a printing ink comprising low boilingsolvents. However, U.S. Pat. No. 4,357,164 relates to a waterlesslithographic printing process, which does not cornprise the use of afount solution, i.e. does not give rise to the fluting problem at all.

It becomes apparent from the various different, still unsuccessful,approaches to reduction and prevention of fluting described in the artthat there exists a strong need for better quality of printed substrateobtained by heat-set, including HSWO printing processes.

It is therefore an object of the present invention to provide printedsubstrate with superior properties in terms of considerably lessfluting. Still further, the printed substrate preferably also showssuperior gloss and superior smoothness.

Also, an improved heat-set offset printing process for obtaining aprinted substrate with achieving less fluting is an object of thisinvention.

The objects of the invention are solved by the subject-matter describedin the patent claims.

The invention relates to a printed substrate comprising a substrate anda printing ink-characterized in that at least one side of the substratehas a surface with such porosity that the Gurley-Hill air permeabilityvalue of the substrate is above 7000 s/100 ml, and it comprises anprinting ink formulated as heat-set offset printing ink with reducedtack comprising at least one pigment and a mixture of at least twosolvents boiling in the range of from 200° C. to 270° C.

According to an embodiment of the invention at least on solvent presentin the ready-to-use printing ink boils in the range of from 210° C. to230° C. At least one further solvent being present in the ready-to-useprinting ink boils in the range of from 240° C. to 270° C. The mixtureof at least two solvents being present in the ready-to-use printing inkcomprise solvents that may have different aromatic contents.

Preferably, the pigment present in the ready-to-use printing ink isadded in the form of a pigment paste.

According to an embodiment of the invention at least one solvent presentin the ready-to-use printing ink has an aromatic content of 1% byweight, based on the solvent, and at least a second solvent beingpresent in the ready-to-use printing ink has an aromatic content of 12%by weight, based on the solvent. Preferably at least one solvent beingpresent in the ready-to-use printing ink is a mineral oil solvent.

According to most preferred embodiments of the invention the printedsubstrate comprises a solvent, which solvent was present in theready-to-use printing ink in amounts of at least 1% by weight, based onthe printing ink composition. All solvents present in the ready-to-useprinting ink can be mineral oil solvents.

According to most preferred embodiments of the invention theready-to-use printing ink comprises a varnish in an amount of from 15 to30% by weight based on the ready-to-use printing ink. The varnish cancomprise at least one self-structured resin. The varnish can be free ofgelling agents. Preferably at least one varnish within the printing inkis free of gelling agents.

The porosity of the substrate surface of the printed substrate accordingto the invention can be achieved by coating. The coating can comprise acoating colour comprising at least one coating pigment and/or at leastone bonding agent. The weight ratio of coating pigment to bonding agentcan be in the range of from 1:100 to 100:1. According to preferredembodiments of the invention at least one coating pigment is selectedfrom the group consisting of kaolins, talcs, gypsum and carbonates. Theamount of the coating pigment can be in the range of from 70 to 90percent by weight based on the coating colour. The coating pigment canbe a plate shaped coating pigment.

According to preferred embodiments of the invention the particle sizedistribution of the coating pigment is such that at least 40% by weightof the coating pigment particles are smaller than 2 μm. The particlesize distribution of the coating pigment can be such that at least 25%by weight of the coating pigment particles are smaller than 1 μm andalso can be such that the particle size distribution of the coatingpigment is such that at least 10% by weight of the coating pigmentparticles are smaller than 0.5 μm.

According to the invention the printed substrate has an AFT value ofbelow 0.05%, preferably below 0.03%. The AFT value is measured on apiece of printed substrate in the dimensions of 27 cm×5 cm at standardconditions with an AFT-meter, as set out in more detail below.

The invention also refers to a heat-set offset printing processcomprising

applying to a substrate with such porosity that the Gurley-Hill airpermeability value of the substrate is above 7000 s/100 ml an printingink formulated as heat-set offset printing ink with reduced tackcomprising at least one pigment and a mixture of at least two solventsboiling in the range of from 200° C. to 270° C., andachieving an AFT value of the printed substrate of below 0.05%.

Again, said AFT value is measured on a piece of printed substrate in thedimensions of 27 cm×5 cm at standard conditions with an AFT-meter.

The process according to the invention makes use of an printing ink asdescribed herein, in particular the printing ink comprises as solvents amixture of at least two solvents comprising solvents having differentaromatic contents. As said before, at least one solvent being present inthe ready-to-use printing ink has an aromatic content of 1% by weight,based on the solvent, and at least a second solvent being present in theready-to-use printing ink has an aromatic content of 12% by weight,based on the solvent.

In the process according to the invention the porosity of the surface ofthe substrate used within the process can be achieved by coating. Thecoating can comprise a coating colour comprising a coating pigmentand/or at least one bonding agent.

The invention also refers to a use of a substrate with such porositythat the Gurley-Hill air permeability value of the substrate is above7000 s/100 ml, and a printing ink formulated as heat-set offset printingink comprising at least one pigment and a mixture of at least twosolvents boiling in the range of from 200 to 270° C. for producing aprinted substrate.

The ready-to-use printing inks as used according to the invention maycomprise a mixture of at least two solvents having different aromaticcontents. At least one solvent present in the ready-to-use printing inkscan have an aromatic content of 1% by weight, based on the solvent, andat least a second solvent present in the ready-to-use printing inks hasan aromatic content of 12% by weight, based on the solvent.

It is surprising that the printed substrate according to the presentinvention shows considerably less fluting. The fluting is determined byway of AFT value determination as set out further down below in detail.The term “considerable less fluting” and the term “non-fluting” areherein used synonymously. The highly advantageous and desirablenon-fluting characteristics of the printed substrate according to theinvention are achieved by making use of a substrate, in particular apaper, having a surface as described herein. Still further theparticular printing inks as described before are essential for providingthe printed substrate according to the invention. Finally, of course theheat-set printing process in accordance with the invention makes itpossible to obtain the printed substrate with the advantageousnon-fluting properties.

According to the present invention a printed substrate is provided,which may include a paper made from a fiber-based raw material, whereinat least one side has a surface with such porosity that the Gurley-Hillair permeability value of the substrate is above 7000 s/100 ml,preferably above 10000 s/100 ml and most preferably above 12000 s/100ml.

Gurley-Hill measurement is based on ISO 5636-5: 2003 standard. The airpermeability through substrate is determined, while a higher value (ins/100 ml) corresponds to lower porosity of the sample. The skilledperson is well aware of the process of Gurley-Hill measurement and thementioned respective standard.

The printed substrate furthermore comprises a printing ink formulated asheat set printing ink comprising a mixture of at least two solvents, asdescribed above. The fluting of the printed substrate is reduced, i.e.it shows low fluting (non-fluting) properties. Preferably the AFT valueis reduced more than 40%, preferably more than 50%, compared to therespective AFT value for printed substrate comprising a standard paperand a standard printing ink. The terms “non-fluting” and low flutingpreferably comply with the definition in terms of AFT-values being below0.05% or even below 0.03%.

FIG. 1 illustrates principle of AFT measurement.

FIG. 2 shows an apparatus for AFT measurement.

The AFT device as shown in FIG. 2 is used by insertion of a sample ofprinted substrate of certain length and width into the clamps of the AFTdevice and the shadow pattern which is caused by fluting is observedvisually in low angle light. The substrate, in particular the paper, isstretched until it becomes totally flat, i.e. until the shadow patterncaused by fluting has disappeared. The elongation of the strip bystretching is measured and is the length B minus length A. The AFT valueis the elongation in mm divided by 250 mm (initial length of the samplebetween the clamps into stretching direction) expressed in percent. Forthis method the following further parameters were set:

Sample preparation for AFT measurement:

-   -   Printed paper was obtained by HSWO printing.    -   Fluting was measured on a green area printed on both sides.    -   Yellow optical density was 1.2 and cyan optical density was 1.5        in the printed green area.    -   Dimension of printed green area are 20 cm*20 cm (MD*CD)¹. The        printed area was surrounded by unprinted white paper.    -   27 cm*5 cm (CD*MD)¹ sample is cut over the printed green area.        ¹MD=machine direction, CD=cross machine direction.

Typical AFT values of commercial heat-set offset printed papers are0.07-0.11% (see example below). A sample is considered non-fluting, ifthe AFT value is 0.05% or lower. Measurements are done at 23° C. andrelative humidity of 50%.

Standard paper and standard printing ink in terms of this invention are,for example:

Paper: Commercially available double coated MWC paper. Grammage: 80g/m². Gurley-Hill: 3320 s/100 ml. AFT value 0.085% by using standardprinting ink and web exit temperature 130° C. Base paper of thiscommercial MWC paper is a wood containing base paper with a grammage of45 g/m². The paper is precoated with calcium carbonate coating pigment,binder and additives. Top coating is based on clay and calcium carbonatecoating pigment blend, binder and additives.

Ink: Commercially available heat-set offset printing ink. This printingink is suited for printing on coated paper especially Light WeightCoated paper (LWC) and Super Calendared (SC) paper.

There are no hints in the prior art on how to provide printed substratehaving such high quality in terms of non-fluting properties defined byAFT values after heat-set offset printing.

In particular, the idea that at least one side of the substrate has asurface with such porosity that the Gurley-Hill air permeability valueof the substrate is above 7000 s/100 ml, preferably above 10000 s/100 mland most preferably above 12000 s/100 ml, has never been disclosed. Thesame is true for a printed substrate having the superior properties notonly in terms of non-fluting but also regarding improved gloss andsmoothness. Still further, due to the low tack behaviour of the printingink, the undesired consequences of picking and piling can be avoided.

The present invention provides superior printed substrate obtained by aheat-set printing process, in particular HSWO printing process. Usingthis technique, a very high printing quality with regard to accuracy ofthe image is achieved. Additionally, the printed substrate can beobtained at favorable costs. Any conventional heat-set offset printingmachine can be used in order to prepare the printed substrate of thepresent invention.

For example, coating technologies can be used to obtain two, three,four, five or more surfaces. Such technologies comprise the use of ametering size press, a blade coater with jet applicator, a blade coaterwith roll applicator, a blade coater with SDTA (short dwell timeapplicator), a spray coater or curtain coater or the like or anycombination of these.

The present invention provides superior printed substrate obtained by aheat-set offset printing process. Using this technique, a very highprinting quality with regard to accuracy of the image is achieved.Additionally, the printed substrate can be obtained at favorable costs.Any conventional heat-set offset printing machine can be used in orderto prepare the printed substrate of the present invention.

The invention is specifically based on paper allowing the porosity ofthe surfaces to be controlled. The paper surface is designed to be denseas defined herein in terms of porosity. Together with the printing inksdescribed herein and their synergistic interaction with the substrate,the new and advantageous printed substrate showing the described andclaimed superior properties is obtained.

According to the present invention, the fluting properties of theprinted substrate in terms of AFT values are reduced, namely more than40% reduction, more preferred more than 50% reduction is achieved whencompared to the respective values for printed substrate comprisingstandard paper and standard printing ink, as described before.

Although the inventors do not wish to be bound to such theories, thenon-fluting behaviour of the printed substrate can be achieved by theuse of printing inks formulated as heat set offset printing inkscomprising a mixture of at least two solvents boiling in the range offrom 200° C. to 270° C. Preferably, at least one solvent which may be amineral oil has an aromatic content of 1% by weight. Nevertheless,suited are also solvents with an aromatic content in the range of fromof 0% by weight to 5% by weight, preferably in the range of from 0.5% byweight to 3% by weight.

According to the invention a second solvent, which again can be amineral oil, has an aromatic content of 12% by weight. Nevertheless,suited are also solvents with an aromatic content in the range of from10% by weight to 18% by weight, preferably 11% by weight to 15% byweight.

According to most preferred embodiments the mixture of at least twosolvents present in the printing ink, which solvents may be mineraloils, has an aromatic content in the range of from 6% by weight to 10%by weight, 7% by weight to 9% by weight and most preferred an aromaticcontent of 8% by weight.

The presence of the low boiling solvent allows for drying temperaturesduring the heat-set printing process being sufficiently low to avoidfluting. Preferred web temperatures in the last zone of the heat-setdryer are between 85° C. and 120° C., more preferred between 90° C. and110° C. and most preferred between 95° C. and 105° C. Suited dryingtemperatures are for example 120° C., 110° C., 100° C., 95° C., 90° C.or 85° C.

Preferred amounts of low boiling solvents usable within the compositionsof the invention are greater than 1% by weight (based on ready to useprinting ink compositions), more preferred greater than 5% by weight(based on ready to use printing ink compositions) and in particular offrom 10 to 40% by weight (based on ready to use printing inkcompositions).

The printing inks allow for a particular tack behaviour, which can becaused by the presence of respective varnishes. The varnishes havesuperior properties, because the above-defined aromatic contents allowfor the presence of particular resins (so called self-structured resins)and hence can be designed such that also the tack of the final printingink is reduced.

It has been found that the presence of the special varnish allowing fora particular tack as set out within this application in the printing inkcompositions makes the same suitable for printing on the coatedsubstrate described herein. Due to interaction of the substrate with theprinting ink, the desired non-fluting behaviour of the printed substrateis achieved. Further, picking and piling can be reduced or even avoided.

As mentioned before, by use of the mixture of solvents, such as mineraloils, with different aromatic contents and preferred boiling ranges, thetack of the printing inks is reduced.

The low tack described herein means that at the tack is at least 10%lower than the tack of known non-fluting heat-set offset printing inks,preferably between 10% and 20% lower, more preferably between 12.5% and17.5%, most preferably 15% lower.

According to ISO 12634:1996(E) tack is defined as: “Restoring forcebetween two rotating rollers of a given width caused by the splitting ofan printing ink or vehicle film on the roller surfaces.”

A definition mentioned in the ASTM standard for tack measurement: D4361-97: “Tack—a function of the force required to split a thin fluidfilm of a printing ink or vehicle between two rapidly separatingsurfaces; it is a rheological parameter indicative of internal cohesionof the fluid.”

Tack of printing inks controls their high speed transfer properties. Itmay also be meaningful as to the ability to predict paper picking andwet trapping in multi colour printing. Conventional instrumentsdetermine the force exerted on a measuring roller that is positioned onthe printing ink film of a driving roller.

FIG. 3 shows the construction of a traditional three-roller-tackmeter.

Different manufacturers of tackmeters have established their ownarbitrary scales. In this invention a Tack-o-scope® (Testprint BV,Netherlands.) is used to measure tack.

Operation principle of tack measurement instruments is described below:

A defined weight of printing ink is placed on a three roller system. Theroller system consists of a middle, metallic driving roller, an printingink distribution roller and a measuring roller for tack determination.These two outer rollers are covered with an elastomer layer. After speedadjustment and temperature stabilization the axial force on themeasurement roller is determined. This axial force is used as indicationof tack. The higher the axial force, the higher the determined tacknumber.

The moisture gradient, i.e. the drying gradient, between the printed andunprinted surfaces is minimized, and fluting in heat-set printing, inparticular in the heat-set web offset printing processes, issignificantly reduced or even avoided with the substrate according tothe invention. The obtained density makes it possible to minimize themoisture gradient for the substrate. Drying of the non-imaged areasrelative to the imaged areas is delayed due to the dense substrate,thereby the moisture gradient is reduced, e.g. in the HSWO dryingprocess. By reducing the moisture gradient, fluting is reduced.

In this context, substrate refers to any fiber-based paper, cardboard orfiber product, or the like. Paper can be made from chemical pulp,mechanical pulp, chemimechanical pulp, recycled fiber and the like aswell as mixtures thereof. The paper can be in the form of a paper web,pressweb, or sheets, or another form appropriate for its purpose. Thepaper can contain the proper filling and admixing materials.

In one embodiment of the invention, the surface of the base paper[stock] is provided with a coating colour comprising a coating pigmentand/or bonding agent [adhesive]. Preferably, the weight ratio of coatingpigment to bonding agent is in the range of from 1:100 to 100:1, morepreferably 5:1 to 1:5. In one embodiment, the coating pigment is chosenfrom the group consisting of kaolins, talcs, calcium carbonates, gypsum,their mixtures and similar coating pigments. Preferred coating pigmentsare kaolins and talcs.

Typically there are 100 parts of coating pigment present in the coatingformulation. The proportion of binder in the composition is typicallyfrom 10 to 15 parts, with parts meaning parts per weight.

According to a preferred embodiment, the coating colour comprises aplate-shaped coating pigment, wherein the amount of plate-shaped coatingpigment is more than 70 parts, preferably more than 90 parts.

According to a further embodiment of the present invention, the coatingcolour comprises a spherical coating pigment.

The coating colour can comprise a bonding agent. According to apreferred aspect of the present invention, the bonding agent comprises alatex having a glass transition temperature in the range of from −30 to35° C., more preferably in the range of from 0 to 25° C. The glasstransition temperature can be determined by Differential ScanningCalorimetry (DSC), which is known for the person skilled in the art.

In a further embodiment, the bonding agent is chosen from the groupconsisting of starches, proteins, latexes, carboxy-methyl cellulose,polyvinyl alcohol, their mixtures, and the like. Preferred bondingagents are latexes.

In one advantageous embodiment, the porosity of the paper surface iscontrolled by a coating colour comprising a combination of binder andcoating pigments. In one embodiment, the coating colour comprises 75 to95 percent by weight of coating pigments. The coating pigment can beincorporated into a dispersion, and may appear as a composition, inwhich the coating pigments constitute 50 to 100 percent by weight ofsaid composition. In one embodiment, the bonding agent constitutes 5 to25 percent by weight of the total coating colour. The bonding agent canbe added in form of a solution, and may appear as a composition, inwhich the bonding agent constitutes 10 to 100 percent by weight.

For surface treatment of the paper, any suitable bonding agent andcoating pigment may be used. Moreover, for surface treatment, e.g.admixing materials that are suitable and known per se within the fieldmay be added to the coating colour. The mixture may comprise 0 to 10percent by weight of admixing materials.

In one embodiment of the invention, the paper is calendered, preferablyafter surface treatment.

In the paper production process according to the invention, suitablefiber-based base paper is used as so-called base paper [stock], printingpaper according to the invention can be created, e.g., Light WeightCoated (LWC), Medium Weight Coated (MWC), Machine Finished Coated (MFC),Wood Free Coated (WFC) paper, or similar paper.

In one embodiment, filler materials, coating pigments, bonding agentsand/or other chemicals are added to the fiber furnish in the paperproduction process. Any substances or chemicals known in the field maybe used as filler materials, coating pigments, bonding agents andchemicals.

The coating colour comprised in the paper according to the presentinvention preferably comprises a coating pigment with such particle sizedistribution that at least 40% by weight of the coating pigmentparticles are smaller than 2 μm, at least 25% by weight of the coatingpigment particles are smaller than 1 μm, and 10% by weight of thecoating pigment particles are smaller than 0.5 μm. Particle size meansto be the equivalent spherical diameter determined by a sedimentationtechnique (measured by using Sedigraph 5100).

According to one embodiment of the invention the substrate is surfacetreated with multi-layer treatment; the substrate can be double- ormulti-coated. According to a preferred embodiment it is sufficient thatonly one of the two or more surface treatment layers have the desiredlow porosity.

According to one embodiment of the invention the substrate is surfacetreated by coating and/or surface sizing and the substrate is coated orsurface sized with at least two layers one upon the other. According toa preferred embodiment of the invention the lowest layer adjacent to thesubstrate is coated.

According to one embodiment of the invention the substrate is surfacetreated by pre-coating the one or both surfaces of the substrate.According to one embodimeat the substrate is pre-coated with densepre-coating. According to the preferred embodiment the substrate isfurther coated with at least one more coating layer, which may be denseor not.

The dense surface treatment layer can be any of the two or more surfacetreatment layers. According to one embodiment the dense layer is theuppermost surface treatment layer. According to another embodiment thedense layer is the intermediate surface treatment layer. According toanother embodiment the uppermost surface treatment layer is not thedense layer, which enables a possibility to arrange, for example, betterprintability to the uppermost surface treatment layer.

According to one embodiment the thickness of the at least one densesurface treatment layer is 1 to 14 gsm (grams per square meter) per oneside of the substrate, preferably between 7 to 14 gsm per one side.According to one embodiment the total coating thickness is 7 to 40 gsmper side.

The heat set offset printing process as described and claimed hereinmakes use of the printing inks and substrate as set out within thisdescription. The process according to the present invention runsperfectly stable. The final product has a superior gloss and smoothnessbesides the desired AFT value.

The mineral oil solvents used in this invention are characterized by theboiling ranges, the aromatic contents and the aniline points. Theboiling or distillation range [in ° C.] is determined by distillationaccording to DIN ISO 3405 or ASTM D 86. The initial and the finalboiling points determine the boiling range. The aniline point [in ° C.]describes the solubility power of a solvent and is determined by DIN ISO2977 or ASTM D 611. The aromatic content [in wt %] is determined by ASTMD 2140 or EC-A-A07 (UV). The Hydrocarbon type analysis is done by DIN51378 and determines the content of aromatics (Car), naphthenics (Cn)and paraffinics (Cp) [in %].

Solvent Types Used in this Invention:

Carbon-type composition distillation range Aniline point (DIN 51378,(DIN ISO 3405, (DIN ISO 2977, ASTM D 2140) Component ASTM D 86) ASTM D611), Car Cn Cp Mineral oil A 280-310° C. 82 12 22 66 Mineral oil B240-270° C. 84 <1 25 74 Mineral oil C 210-230° C. 84 <1 <1 99 Mineraloil D 240-270° C. 72 12 22 66

The cloudpoint temperature is a characteristic property, which isdefined as the temperature at which a liquid (solution of a solidmaterial and a solvent) begins to become cloudy. The solubility orcompatibility of resins and varnishes is determined by cloud pointmeasurements using a Chemotronic® device (Novomatics GmbH/Germany).

Furthermore, the present invention refers to the use of varnishes andprinting inks as defined herein within a heat-set offset printingprocess.

The varnishes further comprise vegetable oil, preferably stand oil.

The varnishes preferably comprise a mixture of a main resin and aco-resin.

Preferably, the main resin is a self structured phenolic modified rosinresin with a viscosity of 30 Pas (35% in 6/9 AR blend*) and goodcompatibility (cloud point: 120° C. (in 6/9 AF new*). In addition selfstructured resins have non Newtonian flow behaviour. A skilled personcan determine the structure of a resin by viscosity measurements on arotational viscometer using p-Ostwald method. Standard Newtonian resinshave a p-Ostwald factor of 0.9 to 1.0. Self structured resins have ap-Ostwald factor of 0.6 to 0.8. Examples for preferred self-structuredphenolic modified rosin resins according to the present invention aree.g. Cray Valley Tergraf UZ87, Hexion Setaprint P7950, Arez PM1235.

Preferably, the co-resin is a hydrocarbon resin with a viscosity of 40Pas (55% in 6/9*) and very good compatibility (Cloud point: 110° C. (in6/9 AF*). Examples for preferred hydrocarbon resins according to thepresent invention are e.g. Neville Nevprint LG or Resinall R260.

The printing ink composition comprises a pigment paste in amounts ofbetween 1% and 60% by weight, preferably between 25% and 50% by weightand most preferably between 30% and 45% by weight.

The varnishes are present within the non-fluting heat-set printing inkcomposition described herein in amounts of between 15% by weight and 50%by weight, preferably 15% by weight to 30% by weight, based on theprinting ink composition.

EXAMPLE 1 Varnish “7131”

Resins used in offset technique are characterized by their solubility(Cloud point) and viscosity of the resin in a mineral oil distillatesolution. These solutions can be prepared in Thermotronic® (NovomaticsGmbH/Germany) varnish mixer. Depending on resin type, mixtures are madecontaining between 35% by weight and 55% by weight of hard resin and 45%by weight to 65% by weight mineral oil distillate with aromatic contentadjusted to resin solubility (Testoils e.g.: DOW/Haltermann PKWF 6/9,6/9AF, 6/9AFnew, 6/9AR, 6/9AR blend). Resin solution viscosities aredetermined by a rotational viscometer using a cone (25 mm diameter) anda plate at 23° C. The gap between cone and plate must be 0.05 mm. Theviscosity is measured at a shear rate of 25 s⁻¹.

The varnish comprises a mineral oil with a low boiling range of from 240to 270° C. (one part with an aromatic content of 15%, the other partwith an aromatic content of 1%; the mixture is used to get a finalaromatic content of 8%). It comprises vegetable stand oil. The mainresin is a self structured phenolic modified rosin resin with aviscosity of 30 Pas (35% in 6/9 AR blend*)) and good compatibility(Cloud point: 120° C. (in 6/9 AF new*)). The co-resin is a hydrocarbonresin with a viscosity of 40 Pas (55% in 619*) and very goodcompatibility (Cloud point: 110° C. (in 6/9 AF*)). The resins arediluted in solvent and additives, heated to 180° C. and stirred for 30min. After rheology and tack of the varnish was checked the varnish wascooled down to 130° C. and discharged.

The following table gives an overview over the ingredients of thevarnish for illustration purposes:

Component Varnish “7131” wt. % Mineral oil B Boiling range 240-270° C.,25.0 Aniline point 84° C., C_(ar) < 1% Mineral oil D Boiling range240-270° C., 20.5 Aniline point 72° C., C_(ar) 12% Vegetable oil Standoil, viscosity: 50 poise 6.0 Self structured Viscosity: 30 Pas (35% in6/9 AR blend), 43.5 rosin resin Cloud point: 120° C. (in 6/9 AF new)Hydrocarbon resin Viscosity: 40 Pas (55% in 6/9), 5.0 Cloud point: 110°C. (in 6/9 AF) Total [wt %] 100.0

The resins are diluted in solvents and additives, heated to 180° C. andstirred for 30 min. After rheology and tack of the varnish were checkedthe varnish was cooled down to 130° C. and discharged.

Varnish “1/3”

The following varnish is enclosed for illustration purposes and is not anew varnish.

The resins are diluted in solvent and additives, heated to 160° C. andstirred for 30 min. Then the gelling agent (diluted in solvent) wasadded and stirred for further 30 min. After rheology and tack of thevarnish were checked the varnish was cooled down to 130° C. anddischarged.

The following table gives an overview over the ingredients of thevarnish:

Component “1/3 varnish” wt. % Mineral oil C Bp 210-240° C., Car < 1%, AP= 84° C. 33.8 Vegetable oil Wood oil 5.0 Plasticizer di-acid-di-ester9.4 Phenolic modified Viscosity: 30 Pas (45% in 6/9AR blend*), 32.4rosin resin Cloud point: 135° C. (10% in 6/9AF*) Phenolic modifiedViscosity: 35 Pas (40% in 6/9AR*), 17.4 rosin resin Cloud point: 115° C.(10% in 6/9*) Gelling agent aluminum chelate complex 0.9 AntioxidantMTBHQ solution 1.5 Total 100

Varnish “4/7”

The following varnish is enclosed for illustration purposes and is not anew varnish.

This varnish comprises a mineral oil with a low boiling range of from240 to 270° C. (aromatic content 15%), vegetable oil (wood oil) and aplasticizer (Di-Acid-diEster). The main resin in the varnish has lowviscosity (30 Pas; 45% in 6/9AR blend*) and good compatibility (Cloudpoint: 135° C.; 10% in 6/9AF*). The co-resin has medium viscosity (35Pas; 40% in 6/9AR*), medium compatibility (Cloud point: 115° C.; 10% in619*) and gel reactivity. The varnish is gelled with an aluminiumchelate complex.

The resins are diluted in solvent and additives, heated to 175° C. andstirred for 30 min. Then the varnish was cooled down to 160° C. and thegelling agent (diluted in solvent) was added and stirred for further 30min. After rheology and tack of the varnish were checked the varnish wascooled down to 130° C. and discharged.

The following table gives an overview over the ingredients of thevarnish:

Component “4/7 varnish” wt. % Mineral oil D Bp 240-270° C., C_(ar) =12%, AP = 72° C. 34.1 Vegetable oil Wood oil 5.0 Plasticizerdi-acid-di-ester 8.5 Phenolic modified Viscosity: 30 Pas (45% in 6/9ARblend*), 32.0 rosin resin Cloud point: 135° C. (10% in 6/9AF*) Phenolicmodified Viscosity: 35 Pas (40% in 6/9AR*), 18.0 rosin resin Cloudpoint: 115° C. (10% in 6/9*) Gelling agent aluminum chelate complex 0.9Antioxidant MTBHQ solution 1.5

The varnishes as described in Example 1 are suited to preparenon-fluting heat-set printing ink compositions, some of whichcompositions are described in the following Examples:

EXAMPLE 2 Non-Fluting Heat-Set Printing Ink Composition Series 140000

Series 14000D printing ink is based on varnish 7131. The printing inkcomposition was made of pigment paste (pigment level 30%). Mineral oilsolvents with low boiling range (Boiling point (bp)=210-230° C.,Aromatic content (Car)<1%, Aniline Point (AP)=84° C.) were chosen forimproved drying. Mineral oil solvents with medium boiling range andbetter solubility (bp=240-270° C., Car=12%, AP=72° C.) were chosen forimproved roller stability. A high pigment level allows thin printing inkfilms on printing press, which speeds up drying of the printing inks.For improved rub resistance and good coating colour of the paper sheetsPE and PTFE wax pastes were added.

Composition of Printing Ink Series 140000:

Component Yellow Magenta Cyan Black Yellow pigment paste 33 — — —Magenta pigment paste — 40 — — Cyan pigment paste — — 34 — Black pigmentpaste — — — 41 Toner paste 0.5 — — — Varnish 7131 49.5 40 49 44 PE waxpaste 2 2 2 2 PTFE wax paste 1 1 1 1 Antioxidant solution 1 1 1 1Mineral oil D 7 8 7 6 Mineral oil C 6 8 6 5 Total (wt. %) 100.0 100.0100.0 100.0

The components of the printing ink were mixed in a dissolver attemperatures up to 60° C.

EXAMPLE 3 Non-Fluting Heat-Set Printing Ink Composition Series 14000T

Series 14000T printing ink is based on varnish 7131. The printing inkcomposition was made of pigment paste (pigment level 30%). Only mineraloil solvents with medium boiling range (bp=240-270° C.) were chosen forimproved roller stability and lower tack. For better solubility mineraloil solvents with higher aromatic content were used in Cyan. A highpigment level allows thin printing ink films on the printing press,which speeds up drying. For improved rub resistance and good coatingcolour of the paper sheets PE and PTFE wax pastes were used.

Composition of Printing Ink Series 14000T:

Component Yellow Magenta Cyan Black Yellow pigment paste 33 — — —Magenta pigment paste — 40 — — Cyan pigment paste — — 34 — Black pigmentpaste — — — 41 Toner paste 0.5 — — — Varnish 7131 49.5 40 50 44 PE waxpaste 2 2 2 2 PTFE wax paste 1 1 1 1 Antioxidant solution 1 1 1 1Mineral oil B 13 16 — 11 Mineral oil D — — 12 — Total (wt %) 100.0 100.0100.0 100.0

The components of the printing ink were mixed in a dissolver attemperatures up to 60° C.

EXAMPLE 4 Non-Fluting Heat-Set Printing Ink Composition Series 15000

Series 15000 is based on varnish 7131 and on the “4/7 varnish”. Theprinting ink composition was made of pigment paste (pigment level 30%).Mineral oil solvents with low boiling range (bp=210-230° C.) were chosenfor improved drying. Mineral oil solvents with high boiling range(bp=280-310° C., Car=12%, AP=82° C.) were chosen for improved rollerstability. As yellow is printed last, a medium boiling range solvent canbe used (bp=240-270° C.). A very high pigment level allows thin printingink films on the printing press, which speeds up drying of the printinginks. For improved rub resistance and good coating colour of the papersheets a higher amount of PTFE wax paste was used.

Composition of Printing Ink Series 15000:

Component Yellow Magenta Cyan Black Yellow pigment paste 39.2 — — —Magenta pigment paste — 46.5 — — Cyan pigment paste — — 39.8 — Blackpigment paste — — — 48.3 Toner paste 0.6 — — — 4/7 Varnish 23.0 18.122.4 20.5 Varnish 7131 23.0 18.1 22.4 20.5 PTFE wax paste 2.1 2.1 2.12.1 Antioxidant solution 1.0 1.0 1.0 1.0 Mineral oil A — 6.1 4.1 1.9Mineral oil B 4.1 — — — Mineral oil C 7.2 8.2 8.2 5.6 Total (wt. %)100.0 100.0 100.0 100.0

The components of the printing ink were mixed in a dissolver attemperatures up to 60° C.

EXAMPLE 5 Non-Fluting Heat-Set Printing Ink Composition Series 16000

Series 16000 is based on varnish 7131 and on “1/3 varnish”. The printingink composition was made of pigment paste (pigment level 30%). Mineraloil solvents with low boiling range (bp=210-230° C.) were chosen forimproved drying. Additionally, mineral oil solvents with medium boilingrange (bp=240-270° C.) were added for improved roller stability.Extender paste was used to increase solid content of the printing inkand to improve the solid lay of the printing ink on the paper. A veryhigh pigment level allows for thin printing ink films on the printingpress, which speeds up drying. For improved rub resistance and goodcoating colour of the paper sheets a little amount of PTFE wax paste wasused.

Composition of Printing Ink Series 16000:

Component Yellow Magenta Cyan Black Yellow pigment paste 38 — — —Magenta pigment paste — 45 — — Cyan pigment paste — — 39 — Black pigmentpaste — — — 45 Toner paste 0.6 — — — Extender paste 10 10 10 10 1/3Varnish 18 14 19.5 17 Varnish 7131 20.5 14.5 18.5 17.5 PTFE wax paste 22 2 2 Antioxidant solution 1 1 1 1 Mineral oil B 3.4 7.5 7 6.5 Mineraloil C 6.5 6 3 1 Total (wt. %) 100.0 100.0 100.0 100.0

The components of the printing ink were mixed in a dissolver attemperatures up to 60° C.

COMPARATIVE EXAMPLE

AFT values for different known papers and printing inks printed thereonare shown. The printing ink used for all AFT measurements described inthe table below is SunChemical “Challenge Intensive”. The web exit forall measurements is 130° C.

Paper grade Grammage, g/m² AFT value, % LWC 51 0.104 LWC 57 0.109 LWC 650.094 WFC 70 0.088 MWC 80 0.085 WFC 80 0.084 MWC 90 0.086 WFC 90 0.075WFC 100 0.076

Respective trials with printing inks as described herein printed on thepaper according to this invention show the desired non-fluting behaviourof the resulting printed substrate/paper, i.e. AFT values are below0.05%.

1. Printed substrate comprising a substrate and a printing inkcharacterized in that at least one side of the substrate has a surfacewith a porosity such that the Gurley-Hill air permeability value of thesubstrate is above 7000 s/100 ml, and the printing ink is a heat-setoffset printing ink with reduced tack comprising at least one pigmentand a mixture of at least two solvents boiling in the range of from 200°C. to 270° C.
 2. The printed substrate according to claim 1, wherein thesurface of the substrate is coated with at least two layers, at leastone of which is said printing ink.
 3. The printed substrate according toclaim 1, wherein at least one solvent present boils in the range of from210° C. to 230° C.
 4. The printed substrate according to claim 1,wherein at least one solvent boils in the range of from 240° C. to 270°C.
 5. The printed substrate according to claim 1, wherein the mixture ofat least two solvents comprise solvents having different aromaticcontent.
 6. The printed substrate according to claim 1, wherein thepigment is in the form of a pigment paste.
 7. The printed substrateaccording to claim 1, wherein at least one solvent present has anaromatic content of 1% by weight, based on the solvent, and at least asecond solvent present has an aromatic content of 12% by weight, basedon the solvent.
 8. The printed substrate according to claim 1, whereinat least one solvent present is a mineral oil solvent.
 9. The printedsubstrate according to claim 1, wherein the heat-set offset printing inkcomprises at least one low boiling solvent in an amount of from 1 to 40%by weight, based on the printing ink composition.
 10. The printedsubstrate according to claim 1, wherein all solvents present are mineraloil solvents.
 11. The printed substrate according to claim 1, whereinthe printing ink comprises a varnish in an amount of from 15 to 30% byweight based on the printing ink.
 12. The printed substrate according toclaim 11 wherein the varnish comprises at least one self-structuredresin.
 13. The printed substrate according to claim 1, wherein thesubstrate surface having said porosity is a coated surface.
 14. Thesubstrate according to claim 13, wherein the coating comprises a coatingcolour comprising at least one coating pigment, at least one bondingagent or both.
 15. The printed substrate according to claim 14, whereinboth are present and the weight ratio of coating pigment to bondingagent is in the range of from 1:100 to 100:1.
 16. The printed substrateaccording to claim 14, wherein at least one coating pigment is selectedfrom the group consisting of kaolins, talcs, gypsum and carbonates. 17.The printed substrate according to claim 14, wherein the amount of thecoating pigment is in the range of from 70 to 90 percent by weight basedon the coating colour.
 18. The printed substrate according to claim 14,wherein the coating pigment is a plate shaped coating pigment.
 19. Theprinted substrate according to claim 14, wherein the particle sizedistribution of the coating pigment is such that at least 40% by weightof the coating pigment particles are smaller than 2 μm.
 20. The printedsubstrate according to claim 14, wherein the particle size distributionof the coating pigment is such that at least 25% by weight of thecoating pigment particles are smaller than 1 μm.
 21. The printedsubstrate according to claim 14, wherein the particle size distributionof the coating pigment is such that at least 10% by weight of thecoating pigment particles are smaller than 0 5 μm.
 22. The printedsubstrate according to claim 14 wherein the AFT value of the printedsubstrate is below 0.05%.
 23. The printed substrate according to claim14, wherein the Gurley-Hill air permeability value is above 10000 s/100ml.
 24. The printed substrate according to claim 14, wherein theGurley-Hill air permeability value is above 12000 s/100 ml.
 25. Aheat-set offset printing process comprising applying to a substrate thathas a surface with a porosity such that the Gurley-Hill air permeabilityvalue of the substrate is above 7000 s/100 ml a heat-set offset printingink with reduced tack comprising at least one pigment and a mixture ofat least two solvents boiling in the range of from 200° C. to 270° C.and achieving an AFT value of the printed substrate of below 0.05%. 26.The process according to claim 25, wherein the surface of the substrateis coated with at least two layers, at least one of which is saidprinting ink.
 27. The process according to claim 25, wherein theprinting ink solvent mixture comprising solvents having differentaromatic contents.
 28. The process according to claim 25, wherein atleast one solvent present has an aromatic content of 1% by weight, basedon the solvent, and at least a second solvent present has an aromaticcontent of 12% by weight, based on the solvent.
 29. The processaccording to claim 25, wherein the surface of the substrate having saidporosity is a coated surface.
 30. The process according to claim 29,wherein the coating comprises a coating colour comprising a coatingpigment, at least one bonding agent or both. 31-35. (canceled)
 36. Amethod for reducing fluting of a heat-set offset printed substratecomprising applying a heat-set offset printing ink to a coated substrateand thereby achieving an AFT value of the printed substrate below 0.05%.37. The method according to claim 36, wherein the substrate has aporosity of the surface being such that the Gurley-Hill air permeabilityvalue of the substrate is above 7000 s/100 ml.
 38. A method for reducingthe web temperature in the last zone of a heat-set dryer in a heat-setoffset printing process comprising applying a heat-set offset printingink with low boiling solvent to a coated substrate.
 39. The methodaccording to claim 38, wherein the substrate has a porosity of thesurface being such that the Gurley-Hill air permeability value of thesubstrate is above 7000 s/100 ml.
 40. The printed substrate according toclaim 1, wherein the heat-set offset printing ink comprises at least onelow boiling solvent in an amount of from 1 to 5% by weight based on theprinting ink composition.